A sailplane owes its performance to the utilization of energy supplied from external sources. Sensitive rate of climb instruments or variometers connected to static pressure sources are commonly used to indicate rising or sinking of the sailplane.
The ability of a sailplane to remain aloft for long periods of time, or to cover significant cross-country distances, is dependent upon its effective use of energy supplied by external sources. For this reason, clear and accurate information concerning the total energy situation and its rate of change are extremely significant to successful soaring. While there are many factors involved in a rigorous treatment of total energy, it is possible for a pilot to interpret his total energy situation with simple modifications to a variometer system. Several methods have previously been used to provide dynamic pressure compensation for rate of climb when zooming or diving, so that the variometer indicates rate of change in total energy instead of rate of change in potential energy. The most widely used compensators have been of two types: diaphragm-volume systems and venturi systems. However, these systems have not proved adequate in continuous flight to give an immediate reading to the pilot of a sailplane of the total energy available, and of rates of change in total energy.
The total energy of a sailplane at a given time is the sum of its potential and kinetic energies. A pilot customarily determines his total energy situation by a glance at the altimeter and the air speed indicator. In addition to sensing indications of the absolute value of total energy, it is important to "energy management" that the pilot also be able to sense the rate of change in total energy at all times.
The rate of change in total energy is primarily affected by the drag of the sailplane which is constantly reducing the useful energy, and the air mass energy effects on the sailplane. The drag is dependent on the aerodynamic characteristics of the sailplane, its velocity, altitude, and the load factor. There is little a pilot can do about the aerodynamic characteristics of his sailplane in flight; however, he can control the velocity and the load factor affected by maneuvers. The air mass will be producing sink, climb, or velocity increments to the sailplane which are dependent on its characteristics and the pilot's skill in positioning the sailplane with respect to local air currents.
In summary, the useful total energy from a pilot's viewpoint may be thought of as the instantaneous total energy associated with his given altitude and velocity, less the energy being dissipated by the drag of the sailplane moving along its flightpath, plus the energy being added to the sailplane by the air mass.